The present invention relates generally to processing audio signals and more specifically to the reduction or elimination the Doppler effect in loudspeakers.
The Doppler Effect is observed in a passing siren & astronomy (red shift). It is exploited in Doppler radar systems. Indeed, the Doppler Effect is present whenever there is relative movement between an observer and a wave producing source.
It is known that the Doppler effect produces unwanted distortion in audio loudspeakers (aka speakers or loudspeakers). A speaker operates by passing a current through a coil exposed to a magnetic field (usually produced by a fixed magnet). The time varying current in the coil creates a corresponding alternating magnetic field which in turn interacts with the magnet to produce a mechanical force that moves the coil. The coil is mechanically connected to a diaphragm which also moves to produce sound waves through the air.
It is known that the Doppler effect causes distortion in speakers because as the speaker diaphragm moves outwardly (towards the listener) the sound wave is compressed which results in an increase in pitch and conversely a decrease in pitch as the diaphragm moves inwardly (away from the listener). This effect is exacerbated when more than one tone is present. The lower tone modulates the higher tone creating side bands spaced according to the frequency difference. For example, a speaker with tones at 100 & 3 kHz would have side bands at 2900 & 3100 Hz. The side bands are the DD. The degree of Doppler Distortion (DD) is proportional to the difference between the tones. i.e. a greater difference results in more DD. Additionally, the magnitude of the side bands is proportional to the amplitude of the lower frequency signal.
The frequency spectrum of music is diverse. Thus, Doppler distortion is more prevalent and complex in music. Crossover networks can reduce the effect. However, small speakers (e.g. in computer monitors, television sets, and small portable devices) generally do not incorporate crossover circuitry. Thus, there is a need for a system and method of eliminating DD in such systems.
The present invention is useful not only in reducing or eliminating the Doppler effect in loudspeakers, but also in allowing audio signals to be adjusted to create a more pleasing sound to one listening to music. An audio signal is processed by passing it through a speaker, the cone of which is mechanically connected to a moving sleeve. A coil is wrapped around the sleeve. There is a metal shaft displaced inside of the sleeve. The shaft is connected to a magnet. Thus, movement of the speaker cone causes the coil to move through a magnetic field which creates a voltage across the coil. This voltage is proportional to the audio signal with DD. The difference between the original audio signal and the processed signal is DD. This difference is subtracted from the original audio signal to produce an output with minimized or cancelled DD.